30 Journal of Comparative Neurology. 



gin, often exceedingly so. They often continue as extremely 

 fine fibers nearly to or even through the commissure. Usually 

 they thicken into medium or coarse fibers before they enter the 

 commissure, but a considerable number of fine fibers are found 

 in both limbs of the commissure as far as they can be traced. 

 The fibers turn ventrally and caudally after crossing and spread 

 widely through the lateral walls of the mid brain. The diffuse 

 arrangement of the fibers makes it very hard to trace them, 

 especially as they become mingled with the tracts descending 

 from the tectum. I have traced the fibers to about the caudal 

 border of the mid brain and suppose that they are destined to 

 make connections with the motor nuclei of the medulla. The 

 posterior commissure thus constitutes an important, if not the 

 chief path for fibers from the central grey of the mid and 'tween 

 brain to the motor apparatus of the medulla. It thus forms 

 part of the path for carrying descending impulses from the 

 striatum. The cells of the central grey which contribute fibers 

 to the posterior commissure constitute the nucleus of that com- 

 missure, homologous with the nucleus described by Kolliker 

 in mammals (Gewebelehre, p. 445). 



The end-nucleus of Meynekt's bundles, a part of which 

 lies in the central grey, will be described in the following 

 section. 



c. Commissura ansulata (Figs. 13, 14, 30). 



This commissure is not as complex as in Acipenser. I 

 have demonstrated only the tractus tecto-bulbaris and bulbo- 

 tectalis, including the neurites of the spindle cells in the acus- 

 ticum and those of the Purkinje cells in the crebellum. It is 

 altogether probable that the neurites of commissural cells and of 

 the cells of the nucleus of Meynert's bundles are present also. 



C. ' T%veen Brain. 



a. Ganglia habenulae, epiphysis, and bundles of Mey- 

 nert (Figs. 14-17, 30). 



The general relations of these structures have been suffici- 

 ently described above. The greater size of the right ganglion 

 habenulae is due to greater volume of both cells and fibers. 



